Page 165 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

144 Maritime Archaeology: A Technical Handbook, Second Edition
With the completion of the survey of the 1999 area, three transponders, T3, T4, and T5, were moved to incorporate the new stone area discovered northeast of the 1999 grid, approximately 12–15 m from T2. In the Site Sur- veyor project these were renamed: T3 = T7; T4 = T8; and T5 = T9. Because the survey needed to be extended, three transponders were kept in their original position and the other three moved to encompass the new area. The newly positioned transponders were leveled between 3.5 and 4m, slightly deeper than the initial survey as this site was in the shipping channel. Two tapes approximately 25 m in length extended in a NS orien- tation delineating the new stone area. The ends of each of these tapes were buoyed and the transponders placed at these positions.
Deployment of the HPASS in this area was relatively expeditious with up to 60 points or 400 measurements acquired in a dive. Initial post- processing results showed that HPASS could not only determine the relative positions of the stones, but could also plot the four corners of individual blocks. These macrosurvey capabilities were made possible due to the relatively small intertransponder distances and good geometry between the targets and transponders.
The HPASS data were processed using Site Surveyor 2.1.2. One of the ultimate aims of this survey is to integrate acoustic data with geodetic infor- mation with constructional detail then projected into it (T. J. Maarleveld, personal communication). These results, used in conjunction with terrestrial surveyed positions of the bridge remains, will eventually be used to illus- trate where the bridge crossed the river. Exporting a DXF file to ArcView GIS and then to Freehand so that it could be combined with the manual measurements created the final graphic presentation.
2. Processing in the Field
On site it was found more practical to process each day’s data as a single project. The results showed the relative positions of the targets more clearly without the confusion of several layers of points. The operators were also able to identify potential problems in the survey and to re-measure if necessary. Observing the residuals for individual distances and rejecting measurements with high residuals manipulated data. Measurements with high residuals were probably the result of multiple pathways interference. Results indicated that the main errors were multipaths, because the resid- uals were mainly positive. This means that the acoustic signal, instead of traveling on the straight line path between the transducer and the diver unit (the shortest path), travels along a path where it is reflected from the water surface one or more times. Such paths would be longer than the direct path